1. Field Of The Invention
This invention relates to flash electrically-erasable programmable read-only memory (flash EEPROM) arrays and, more particularly, to methods for reliably storing identifying fields associated with each sector of data stored in such arrays.
2. History Of The Prior Art
Modern computer systems make extensive use of long term memory. Typically this memory is provided by one or more electro-mechanical hard (fixed) disk drives made of flat circular magnetic disks which rotate about a central axis and which have a mechanical arm to write to or to read from positions on the magnetic disk. Hard disk drives are very useful and have become almost a necessity to the operation of personal computers. However, such electro-mechanical drives are relatively heavy, require a significant amount of space within a computer, require a significant amount of the power in use, and are very susceptible to shock.
Recently, forms of long term storage other than electro-mechanical hard disk drives have become feasible for use in computers. One of these is flash EEPROM. A flash EEPROM memory array includes a large plurality of floating-gate field effect transistors arranged as memory cells in typical row and column fashion with circuitry for accessing the individual cells and placing the memory transistors of those cells in one of two memory conditions. A flash memory cell, like a typical EPROM cell retains information when power is removed; however, the condition of the cells of a flash EEPROM array may be changed electrically while the array is in place in a host. Flash EEPROM memory has a number of attributes which adapt it to use as long term memory. It is light in weight, occupies very little space, and consumes less power than electro-mechanical disk drives. It is especially rugged and withstand without adverse effects repeated drops each of which would destroy a typical electro-mechanical hard disk drive.
Flash EEPROM memory arrays are by their nature extremely reliable. The inherent reliability of transistors compared to magnetic media causes this to be true. It has been estimated that while electro-mechanical hard disk drives average a raw media failure once every 2.sup.14 bits, flash EEPROM arrays will produce such an error approximately once every 2.sup.19 bits. Thus, flash EEPROM array is substantially less likely to fail than are electro-mechanical hard disk drives. For this reason, there have before this time been no attempts to enhance the reliability of the storage of data in such arrays. Now, however, attempts are being made to ensure the reliability of the data stored in flash EEPROM array. For example, a method of improving read reliability is disclosed in U.S. patent application Ser. No. 07/969,756, entitled Method and Apparatus to Improve Read Reliability in Semiconductor Memories, S. Wells et al, filed Oct. 30, 1992, and assigned to the assignee of the present invention.
However, the information stored in any long term storage array includes not only the data itself but a field stored with each sector of data to identify that data so that it may be used. No known attempt has been made to enhance the reliability of the storage of these identification fields in flash EEPROM arrays.